>
> The CW, QRP, and upper frequency limitations greatly simplify the
amplifier
> electronics. I envisioned a separate opamp-like current source feeding
> directly into the base of each non-resonant vertical element. I am
assuming
> that one can build a clean current source amp that can deliver a few watts
> of 1.8 Mhz CW into a reactive load.
One can probably buy an off the shelf opamp that will do this, or, certainly
cobble up a hybrid with a fast opamp driving some output transistors. Check
out the offerings of Apex. They're expensive, but will give you ideas. If
you're not too worried about DC efficiency, there's a lot of high power
IGBTs and FETs that work at 4 MHz, have enough gain to be useful to be
driven by a decent opamp, etc.
For QRP, the sum of the output of all
> the amps will be 10 watts or less, so you can design with lots of overhead
> and little concern for efficiency. Below 2Mhz, you might be able to think
> more like an audio amp designer than an rf amp designer.
>
> As for pattern generation, I figured that you would try to place your
> elements in a desirable configuration, but if real estate concerns
preclude
> that, you do the best you can. Then you measure the position of each
> element in respect to each other. Then you apply a signal to each element
> one at a time while receiving on each of the other elements. Since you
know
> their positions, you can have the software solve for all the phase delays
> due to coax, etc from the phase measurements in each RX channel.
There are a variety of clever ways to do this. Think in terms of modulating
the element at some reasonable audio frequency (like a diode driven by a
555), and then using a separate receiver to receive both the reference and
the signal from the element. One receiver chain, so the measurement is
matched. (For details, look up modern implementations of Watson-Watt
direction finders).
From that
> you can easily optimize for power levels and phases for tx beamforming in
> any desired direction based on whatever your criteria (i.e. f/b, gain,
null
> in a particular direction, etc.). Similar optimizations can be used for
rx
> beamforming as well.
>
> Naturally, what I just described is too limited to be commercially
> interesting (except for the half dozen 160m/QRP/CW people out there), but
it
> would be fun to play with. It also might give me an edge for 160m QRP.
> Since all the beamforming is being done in the DSP, you are free to form
> more than one beam or null at the same time. You could be forming a
maximum
> gain lobe in the desired direction while interactively nulling strong
> signals from other directions and doing active noise elimination.
Adaptive nulling on receive is where it's at, I think. Here's an
interesting challenge. Given the pervasiveness of interference from power
lines (BPL or not) which is omnidirectional (but not uniform!), can one come
up with some sort of way to adaptively cancel it without requiring an
excessive number of degrees of freedom. It's not a problem that I've seen
addressed in the literature, which tends to concentrate on "point source"
interferers (maybe, though, since such algorithms have some export
controlled applications, it just hasn't been published in the open
literature... think of removing the background noise in sonar processing)
>
> Oh yes, this is Towertalk and not Antennex. One can use this arrangement
to
> phase up a set of existing towers, regardless of how they are arranged
> (within reason). You could augment the towers with additional wire
elements
> and other things to make as many elements as you think you need. Then let
> the system I described figure use the towers and wires for beamforming.
This is exactly what I intend to do as a starting point. Consider the
portable operation scenario (for real CC&R controlled areas). Deploy an
array of portable elements (verticals or what have you) in a quasi random
arrangement, calibrate, and go.
>
> It's a nice pipe-dream, of course. Besides the functional limitations I
> have already described, can anyone see other weaknesses? Is this too far
>off topic? Perhaps we need a Software Defined Antenna email list.
OT is up to the moderator, but I find that this list has really great people
to shoot at your ideas, and the feedback has been particularly useful in
clarifying the overall objective on phased arrays, and in bringing up
potential shortcomings. Here, at least, you get responses (although they may
be painful to read at first). On some other lists, you get a "that's nice..
but I have a tesla transmitter that will broadcast energy for free" or "why
fool with phased arrays when the CFA will solve all your problems". People
on this list have enormous practical experience and are pretty darn cynical,
which is great when you have a new idea. You want those objections to be
raised early and often, if only to figure out how to refute them.
(Now, back to computing the Chu-Harrington limits on performance for an
antenna in a 50x100x30 foot box, so I know what to shoot for)
Jim, W6RMK
>
> Regards,
> Dudley - WA1X
>
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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